(1) Field of the Invention
The present invention relates to image forming apparatuses, such as printers and copiers. In particular, the present invention relates to registration correction techniques according which registration patterns having a predetermined shape are formed on the surface of an image carrier of an image forming apparatus, the amount of the position deviation of each registration pattern is calculated, and the timings for image forming processes such as exposure timing are adjusted according to the results of calculation.
(2) Description of the Related Art
Image forming apparatuses capable of color printing forms a color image generally by overlaying mono-color images in yellow (Y), magenta (M), cyan (C), and black (K) on an image carrier, such as a photosensitive drum or an intermediate transfer belt. Naturally, mono-color images overlaid with a positional deviation relative to one another results in color registration error.
In view of this, such an image forming apparatus performs registration correction in which the amount of positional deviation is calculated for each mono-color image at a predetermined time (at the time of power-on, for example) and the timings for image forming processes such as exposure timing are adjusted according to the results of calculation. Specifically, a plurality of toner image patterns having a predetermined shape are formed one for each color on the image-carrying surface of an image carrier. Each toner image pattern is used to detect the amount of positional deviation of the corresponding color and hereinafter referred to as “registration pattern”. Subsequently, the position of each registration pattern is detected with an optical sensor (such as a reflection-type toner density sensor). Based on the result of the detection, the amount of positional deviation in the registration pattern in each color is calculated, and the registration correction is carried out by adjusting the timings for image forming processes such as exposure timing.
One method having been used for detecting the position of a registration pattern employs a reflection-type toner density sensor provided with a light-emitting element and a regular reflection photodetector. The toner density sensor irradiates the registration pattern with light emitted from the light-emitting element and detects regular reflection light from the registration pattern (see Patent Literature 1: Japanese patent application publication No. 2001-312116). Another method employs a reflection-type toner density sensor provided with a light-emitting element and a diffuse reflection photodetector to detect diffused reflection light with the diffuse reflection photodetector.
Since the amount of diffuse reflection light is smaller than that of regular reflection light, the latter method compares unfavorably to the former method in detection accuracy. Also, in order to increase the amount of diffuse reflection light to make the latter method comparable to the former method in accuracy, a large light source needs to be employed as the light-emitting element and therefore the manufacturing cost needs to be increased as well. In addition, the latter method is susceptible to various factors, such as the surface condition and color of the image carrier, and therefore has restrictions on detection conditions.
On the other hand, the former method does not require a large light source as the light-emitting element and ensures that an amount of regular reflection light sufficient to detect the position of a registration pattern is obtained by irradiating the registration pattern with a small amount of light. In addition, the former method is not affected by to such factors as the surface condition and color of the image carrier, and therefore has fewer constrains on detection conditions. In view of the above, the former method is preferable as a method for position detection of the registration pattern. As one example, Patent Literature 1 mentioned above discloses a technique for detecting the position of a registration pattern to carry out registration correction using the former method.
According to the technique, the positional deviation of the registration pattern is accurately calculated and the timings for image forming processes such as exposure timing are appropriately corrected based on the results of calculation, which ensures that a color image formed on the image carrier is without color registration error.
In many cases, a reflection-type toner density sensor used for the position detection of a registration pattern is also used for the toner density measurement of patch toner images (of the respective colors of Y, M, C, and K) to adjust image density in image stabilization control. FIG. 11A shows an output characteristic curve plotted between the output voltage and the toner density of a K-color patch toner image measured by a reflection-type toner density sensor that detects regular reflection light.
As shown in the figure, in the case of the color K, the toner image density measured by the reflection-type toner density sensor which detects regular reflection light exhibits such a curve that the sensor output voltage decreases as the toner density increases from the low density to the high density. It means that the reflection-type toner density sensor that detects regular reflection light is usable also for the toner density measurement of a K-color patch toner image.
FIG. 11B shows an output characteristic curve plotted between the output voltage and the toner density of a color patch toner image such as Y, M, or C measured by a reflection-type toner density sensor that detects regular reflection light. In the figure, the reference sign P denotes the output characteristic curve obtained by measurement by the reflection-type toner density sensor which detects regular reflection light, whereas the reference sign Q denotes the output characteristic curve obtained by measurement by the reflection-type toner density sensor that detects diffuse reflection light.
As seen from the output characteristic curve P, in the low-density range, the output voltage of the sensor detecting regular reflection light decreases as the toner density increases. Then, as the toner density increases, the output voltage of the sensor detecting diffuse reflection light gradually increases as shown in the output characteristic curve Q. Being influenced by this increase, in the high-density range, the output characteristic curve P gradually increases rater than decreasing, with the increase of the toner density (see a portion encircled in a dotted line in FIG. 11B). It means that in the case of colors such as Y, M, and C, other than K (colors other than K is hereinafter simply referred to as a “color”), the reflection-type toner density sensor that detects regular reflection light is unable to accurately detect the toner density in a high-density range. Therefore, in view of measurement accuracy, it is not preferable to use a toner density sensor for measuring regular reflection light also for the toner density measurement of a color patch toner image.
One attempt to eliminate the influence of diffuse reflection light is to use a reflection-type toner density sensor having a light-emitting element and two photodetectors, one for regular reflection light and the other for diffuse reflection light. By detecting the difference between regular reflection light and diffuse reflection light, the output characteristic curve as shown in FIG. 11C is obtained in which the sensor output voltage decreases as the toner density increases all the way to the high-density range.
FIG. 11C shows output characteristic curves each plotted between the output voltage and the toner density of a color (i.e., Y, M, or C) patch toner image measured by (i) a reflection-type toner density sensor that detects the difference between regular reflection light and diffuse reflection light, (ii) a reflection-type toner density sensor that detects regular reflection light, and (iii) a reflection-type toner density sensor that detects diffuse reflection light.
In the figure, the reference sign P′ denotes the output characteristic curve of the reflection-type toner density sensor that detects regular reflection light, the reference sign Q′ denotes the output characteristic curve of the reflection-type toner density sensor that detects diffuse reflection light, and the reference sign R denotes the output characteristic curve of the reflection-type toner density sensor that detects the difference between regular reflection light and diffuse reflection light.
As in the output characteristic curve R, by detecting the difference between regular reflection light and diffuse reflection light, the resulting output characteristic curve shows that the sensor output voltage decreases as the toner density increases all the way to the high-density region.
The above observation means that the reflection-type toner density sensor that detects the difference between regular reflection light and diffuse reflection light is usable also for the toner density measurement of a patch toner image in color, such as Y, M, or C.
In one example, Patent Literature 2 (Japanese Patent Application No. 2008-139592) discloses a reflection-type toner density sensor provided with a light-emitting element as well as both a photodetector for regular reflection light and a photodetector for diffuse reflection light. This sensor detects components of both regular reflection light and diffuse reflection light and outputs the difference between the respective components. Patent Literature 2 also discloses the use of the reflection-type toner density sensor for both the toner image measurement of a color patch toner image and the misregistration detection.
Unfortunately, however, the use of reflection-type toner density sensor having two photodetectors for the two purposes as in the conventional technique described above has the following setback. That is, since at least two reflection-type toner density sensors are required for detecting the position of a registration pattern, the number of photodetectors needs to be increased by two as compared with a reflection-type toner density sensor having one photodetector (i.e., a reflection-type toner density sensor having a photodetector for regular reflection light only). This causes increase of the manufacturing cost.